Field of the Invention
The present invention is related to an anti-reflection film and an optical element having the same, which is preferably used in an optical system such as a digital camera, video camera, or TV camera.
Description of the Related Art
Many optical elements such as lenses and filters included in optical systems are fabricated using transparent material (substrate) such as optical glass and optical plastic. If the refractive index for such substrate is increased, the reflectivity of the incident surface and the light exiting surface (light incident and exiting surface) also increases. If an optical element with a light incident and exiting surface having high reflectivity is used in an optical system, the effective amount of light reaching the image is reduced, while at the same time unwanted reflections from the light incident and exiting surface of the optical element enter the image plane and result in ghosting and flaring, which causes a reduction in the optical performance of the optical system. For this reason, an anti-reflection function is applied to the light incident surface of the optical element using a substrate.
Unwanted ghosting and flaring that reflect off the light incident and exiting surface and reach the imaging surface significantly change depending on the form of the optical element and the angle of incidence of light beams to the optical element. For this reason, the anti-reflection film applied to the substrate preferably obtains an excellent anti-reflective effect over a wavelength band as wide as possible at various angles of incidence. Multi-layered anti-reflection films with thin layers of dielectric film applied to the light incident and exiting surface of the substrate by deposition are common as the anti-reflection film applied to the light incident and exiting surface of the substrate. Generally, the anti-reflective effect of the anti-reflection film gradually increases and the wavelength band to prevent reflections widens with higher numbers of layers.
Conversely, if a material having a refractive index lower than the refractive index of 1.38 for magnesium fluoride is used as the material for the deposition film, for example, on the outermost layer of the anti-reflection film (outermost layer to the atmosphere), an anti-reflection function having high performance can be readily obtained. Japanese Patent Laid-Open No. 2012-141594 discloses the use of inorganic materials such as silica and magnesium fluoride and organic materials such as silicon resin and amorphous fluorine resin as materials having a low refractive index. These materials can lower the refractive index by forming voids in the layers. Japanese Patent Laid-Open No. 2012-141594 also proposes an anti-reflection film using a fluorine resin with a refractive index lowered by a factor of approximately 1.3 at wavelengths in a range between 400 to 700 nm.
An anti-reflection film with a 9-layer construction including a magnesium fluoride layer for the topmost layer is also proposed which further reduces the refractive index by a factor of approximately 1.2 at wavelengths in a range between 400 to 700 nm.
In order to reduce the refractive index for a wide band of wavelengths between 400 to 800 nm and obtain an excellent anti-reflection function, it is important to suitably set the refractive index of the substrate, the refractive index and the layer thickness of the thin layer material applied to the substrate, the number of layers, and so forth. Unsuitable configurations are unlikely to result in obtaining an excellent anti-reflective effect at wavelengths over a wide band.